The research was carried out by the National Heart, Lung and Blood Institute (NHLBI), part of the National Institutes of Health.

The study will appear in the August 4 New England Journal of Medicine.

“This important study compared the clinical effectiveness of two drugs with similar mechanisms of action. Doctors and patients need to know the most effective therapy for severe aplastic anemia, a rare life-threatening disorder,” said Susan Shurin, M.D., acting director of NHLBI.

Thymoglobulin is not yet licensed for aplastic anemia in the United States. It has been reported to be effective when used in patients who don’t respond or who relapse following ATGAM treatment. In addition, Thymoglobulin is the only aplastic anemia option available in Europe, Japan, and Latin America. Thus, the findings of this study have implications for management of this disorder internationally.

Aplastic anemia is a rare disorder, newly diagnosed in approximately 1,000 patients in the United States every year. The disease destroys bone marrow and severely lowers the number of functional blood cells in the body. This can lead to anemia, hemorrhaging, and increased risk of infections. It is often fatal if not successfully treated. There are multiple causes of aplastic anemia, including toxic exposures, irradiation, inherited diseases, and autoimmune damage, often following an infection such as hepatitis.

Both agents are immune-suppressing drugs known as anti-thymocyte globulins (ATGs). They are toxic to thymic-derived lymphocytes, or T-cells, which are important part of the immune system that removes infected or diseased cells from the body. ATG may therefore be effective when bone marrow failure is due to an autoimmune attack by T-cells. The difference is that ATGAM is derived from horse plasma, while Thymoglobulin is derived from rabbit plasma.

Thymoglobulin’s past positive results with resistant or relapsed patients suggested that it might be a better choice as a patient’s first aplastic anemia treatment.

However, six months after starting treatment, 68 percent of patients given ATGAM had improved levels of red blood cells, white blood cells, and platelets, compared to 37 percent of patients given Thymoglobulin. After three years, survival was also significantly different. Ninety-six percent of ATGAM patients survived, compared to 76 percent of Thymoglobulin patients.

“This study suggests that ATGAM should remain the first-line regimen of choice in treating severe aplastic anemia,” said NHLBI hematologist Phillip Scheinberg, M.D., a lead author on the study.

The study was designed to compare the two ATG types as treatments for aplastic anemia in whom no non-autoimmune cause was identified. The team enrolled 120 patients from 2 to 77 years of age, and randomly assigned them to either the horse or the rabbit group (60 participants in each).

“It is difficult to do head-to-head studies in rare diseases such as aplastic anemia,” Scheinberg said. “Our ability to carry out this trial underscores the importance of having referral centers, such as the NIH Clinical Center, for rare diseases. This research also shows the need to conduct randomized clinical trials for all drugs and to not just rely on assumptions, even if they are based on seemingly solid evidence. A stronger drug is not necessarily a better drug.”

This important finding may affect the future treatment of aplastic anemia, particularly in places like Europe where ATGAM is not currently used. It may also inspire further studies that compare horse and rabbit ATG to better understand how ATG helps restore bone marrow. Since some patients responded very well to Thymoglobulin and others responded poorly to ATGAM, future work might identify the optimal ATG treatment for specific patient subgroups.

The National Heart, Lung, and Blood Institute (NHLBI) is a component of the National Institutes of Health. NHLBI plans, conducts, and supports research related to the causes, prevention, diagnosis, and treatment of heart, blood vessel, lung, and blood diseases; and sleep disorders. The Institute also administers national health education campaigns on women and heart disease, healthy weight for children, and other topics. NHLBI press releases and other materials are available online at: www.nhlbi.nih.gov.

About the National Institutes of Health (NIH): NIH, the nation’s medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

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